The present invention relates to an exposure controller of a copying machine and more specifically to an exposure controller that can control light exposure either manually or automatically.
An electrophotographic copying machine has charging, exposing, developing, transferring, charge-removing and cleaning devices arranged around a photoreceptor as a recording medium to form an image corresponding to that of an original document onto a copy paper. In the copying process, light exposure is controlled to develop an image of adequate density. The most typical method for the light exposure control is to control the copy lamp light volume in illuminating the original document. The smaller light volume results in a darker image and the larger light volume results in a thinner or lighter image. If the copy lamp light volume is adjusted according to the print density on the original document, a copy with a clear image of an adequate density can be constantly obtained.
Conventionally, the copy lamp light volume is adjusted manually by a light exposure control lever according to the print density of the original document. However, such adjustment by guessing often results in an inadequate light volume adjustment. To avoid such an inadequate adjustment, an automatic light exposure control has been proposed and put into practice. According to the proposed automatic control method, the light reflected by the original document is detected to control the line voltage to be applied to the copy lamp accordingly. Since copy lamp light volume is controlled to be suited to the print density of the original document, the possibility of improper light volume adjustment is eliminated.
In some copying machines, an automatic or manual light exposure control mode can be discretionally selected. With the automatic control mode, the image is copied with the optimal density as long as the print density is uniform over the entire original document. If the print density varies greatly such as the case with a photograph, the copy lamp light volume tends to be automatically set for the darker area of the original document, so the lighter area of the document cannot be copied clearly. In such a case, it is necessary to switch over the automatic control mode to the manual mode, enabling the operator to manually adjust the copy lamp light volume. In the light exposure controller, however, the light volume is automatically set at the medium value when the automatic control mode is switched over to the manual mode. This makes it difficult for the operator to determine which side (the lighter side or the darker side) the light volume should be adjusted to. As a result, the copy lamp light volume can be inadequately adjusted, causing a copy image of improper density. In some light exposure controllers, the light volume is automatically adjusted at the value set for the last manual control mode copying operation when the automatic control mode is switched over to the manual mode. In this case as well, proper light exposure control is difficult for the same reason as above.